How much knowledge of the world is learnt through words?
If I understand correctly, what you are looking for is called "common sense reasoning" in NLP research.

Research in this field revolves around benchmark data sets, where good performance indicates some ability to do common sense reasoning. Here is a nice collection of data sets and research by Sebastian Ruder:

http://nlpprogress.com/english/common_sense.html



In the end, the main question is not


  How much knowledge of the world is learnt through words?


since it is virtually unanswerable if asked in this form. A question that is answered in NLP common sense reasoning research is


  Out of 100 specific decisions that my model needs to take, for how many does the model show the ability to reason correctly?